Researchers propose using distant quasars to test Bell's theorem

February 20, 2014
by Jennifer Chu

In a paper published this week in the journal Physical Review Letters, MIT researchers propose an experiment that may close the last major loophole of Bell's inequality—a 50-year-old theorem that, if violated by experiments, would mean that our universe is based not on the textbook laws of classical physics, but on the less-tangible probabilities of quantum mechanics.

Such a quantum view would allow for seemingly counterintuitive phenomena such as entanglement, in which the measurement of one particle instantly affects another, even if those entangled particles are at opposite ends of the universe. Among other things, entanglement—a quantum feature Albert Einstein skeptically referred to as "spooky action at a distance"— seems to suggest that entangled particles can affect each other instantly, faster than the speed of light.

In 1964, physicist John Bell took on this seeming disparity between classical physics and quantum mechanics, stating that if the universe is based on classical physics, the measurement of one entangled particle should not affect the measurement of the other—a theory, known as locality, in which there is a limit to how correlated two particles can be. Bell devised a mathematical formula for locality, and presented scenarios that violated this formula, instead following predictions of quantum mechanics.

Since then, physicists have tested Bell's theorem by measuring the properties of entangled quantum particles in the laboratory. Essentially all of these experiments have shown that such particles are correlated more strongly than would be expected under the laws of classical physics—findings that support quantum mechanics.

However, scientists have also identified several major loopholes in Bell's theorem. These suggest that while the outcomes of such experiments may appear to support the predictions of quantum mechanics, they may actually reflect unknown "hidden variables" that give the illusion of a quantum outcome, but can still be explained in classical terms.

Though two major loopholes have since been closed, a third remains; physicists refer to it as "setting independence," or more provocatively, "free will." This loophole proposes that a particle detector's settings may "conspire" with events in the shared causal past of the detectors themselves to determine which properties of the particle to measure—a scenario that, however far-fetched, implies that a physicist running the experiment does not have complete free will in choosing each detector's setting. Such a scenario would result in biased measurements, suggesting that two particles are correlated more than they actually are, and giving more weight to quantum mechanics than classical physics.

"It sounds creepy, but people realized that's a logical possibility that hasn't been closed yet," says MIT's David Kaiser, the Germeshausen Professor of the History of Science and senior lecturer in the Department of Physics. "Before we make the leap to say the equations of quantum theory tell us the world is inescapably crazy and bizarre, have we closed every conceivable logical loophole, even if they may not seem plausible in the world we know today?"

Now Kaiser, along with MIT postdoc Andrew Friedman and Jason Gallicchio of the University of Chicago, have proposed an experiment to close this third loophole by determining a particle detector's settings using some of the oldest light in the universe: distant quasars, or galactic nuclei, which formed billions of years ago.

The idea, essentially, is that if two quasars on opposite sides of the sky are sufficiently distant from each other, they would have been out of causal contact since the Big Bang some 14 billion years ago, with no possible means of any third party communicating with both of them since the beginning of the universe—an ideal scenario for determining each particle detector's settings.

As Kaiser explains it, an experiment would go something like this: A laboratory setup would consist of a particle generator, such as a radioactive atom that spits out pairs of entangled particles. One detector measures a property of particle A, while another detector does the same for particle B. A split second after the particles are generated, but just before the detectors are set, scientists would use telescopic observations of distant quasars to determine which properties each detector will measure of a respective particle. In other words, quasar A determines the settings to detect particle A, and quasar B sets the detector for particle B.

The researchers reason that since each detector's setting is determined by sources that have had no communication or shared history since the beginning of the universe, it would be virtually impossible for these detectors to "conspire" with anything in their shared past to give a biased measurement; the experimental setup could therefore close the "free will" loophole. If, after multiple measurements with this experimental setup, scientists found that the measurements of the particles were correlated more than predicted by the laws of classical physics, Kaiser says, then the universe as we see it must be based instead on quantum mechanics.

"I think it's fair to say this [loophole] is the final frontier, logically speaking, that stands between this enormously impressive accumulated experimental evidence and the interpretation of that evidence saying the world is governed by quantum mechanics," Kaiser says.

Now that the researchers have put forth an experimental approach, they hope that others will perform actual experiments, using observations of distant quasars.

Physicist Michael Hall says that while the idea of using light from distant sources like quasars is not a new one, the group's paper illustrates the first detailed analysis of how such an experiment could be carried out in practice, using current technology.

"It is therefore a big step to closing the loophole once and for all," says Hall, a research fellow in the Centre for Quantum Dynamics at Griffith University in Australia. "I am sure there will be strong interest in conducting such an experiment, which combines cosmic distances with microscopic quantum effects—and most likely involving an unusual collaboration between quantum physicists and astronomers."

"At first, we didn't know if our setup would require constellations of futuristic space satellites, or 1,000-meter telescopes on the dark side of the moon," Friedman says. "So we were naturally delighted when we discovered, much to our surprise, that our experiment was both feasible in the real world with present technology, and interesting enough to our experimentalist collaborators who actually want to make it happen in the next few years."

Adds Kaiser, "We've said, 'Let's go for broke—let's use the history of the cosmos since the Big Bang, darn it.' And it is very exciting that it's actually feasible."

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30 comments

in which the measurement of one particle instantly affects another, even if those entangled particles are at opposite ends of the universe

Before anyone asks: Please note the word "measurement". This is fundamentally different from "setting". While both "measurement" and "setting" would allow knowledge about correlation of the particle states only "setting" allows information transmission. Setting of one particle's properties after the entangled objects have been separated does not affect the other one. It just breaks the entanglement. That's why it's called "spooky ACTION at a distance" and not "information teleportation".

on the dark side of the moon

Argh. The moon has no dark side. It has a far side (as seen from Earth).

That's kinda what I was wondering. How sure can anyone be that there's not another loop-hole?

I wouldn't place any bets just yet. I suspect that the final test will have more to do with figuring out something big that we don't know yet.

Besides, isn't this test somewhat circular? They're using the rules of quantum field theory to (in)validate QFT. If might be simply that we don't have QFT quite right yet, somewhat similar to how we thought Newtonian dynamics could model gravity. It's impossible to rule that out, and I think that surely qualifies as a loop-hole.

What if our understanding of subatomic particles is so wrong that we aren't even measuring what we think we're measuring? I'm not saying that's the case, but I think they're being optimistic when they claim there's only three loop-holes.

It's merely measured in a different way. Quantum Mechanics doesn't negate Chemistry. They are different levels. QM is the foundation. Chem is the next level up. Same as with Classical Physics. I think it's about increased resolution. At the macro scale, the rules still apply.

I suppose if we allowed space to have a more complicated structure locality would be harder to define. Shortcuts might be available. These would not be wormholes but simply alternate routes between points, available in certain settings. The speed of light would still be a limit, but the distance traveled would be much shorter. I guess then you could have some information exchanges that would seem to be FTL, and what would appear to be violations of causality. I like to think of such a space as richly interconnected on small scales. Would that work, I wonder?

even if those entangled particles are at opposite ends of the universe.

Right but observe, this produces a paradox:

they would have been out of causal contact since the Big Bang some 14 billion years ago,

But this would lead to a contradiction, since you are basing the measurement on the assumption of light speed communication, but the theory you are trying to prove does not require light speed to be a limit. therefore the assumption on which you have based your measurement contradicts the theory's conclusion.

with no possible means of any third party communicating with both of them since the beginning of the universe—

Through classical means.

an ideal scenario for determining each particle detector's settings.

No, a self-contradiction.

You need to make a prediction based only on quantum measurements and quantum theory.

You can't mix in classical measurements as a basis to a theory you are trying to use to disprove classical measurements.

Statement A is based on a measurement within the framework A'.Framework B' contradicts Framework A'.Observation B uses measurement A as a point of reference.Observation B appears to support Framework B'.Observation B relies on measurements made in framework A'.Conclusion:The results are logically inconsistent, since B' cannot disprove A' while simultaneously relying on a measurement which only holds true it A' is also true.

In this case, Measurement A is the age and distance of the objects you are using in the experiment. You based their age and distance on Relativity and the Speed of Light postulate, but if quantum information can travel instantly to any distance, then the age of distant objects is unreliable, since the information could have somehow teleported much of the distance.

The measurement "A" is the age and distance to the objects used in the Experiment.

This is obtained by a framework of theories, assumptions, and maths all based on Relativity and the Speed of Light postulate.

But if Quantum Theory is true, then you cannot use the Speed of light to date or measure the distance of objects, since the information could somehow be teleporting much of the distance. While this at first seems unlikely, the mere fact that there is no way to rule it out, AND the theory you are trying to prove could potentially allow that, means the original observation would be invalidated if the theory were proven true.

If your only proof of the theory is based on calculations which are themselves based on the (classical) observation of the distance and age, then you have contradicted yourself, since the consequences of the new theory implies that said measurements are not necessarily valid for all cases.

You can always go for something like an Alcubierre drive. While that does not mean that you move faster than the speed of light (the craft doesn't move at all relative to local space) the entire structure moves faster than light. This can be achieved because the expansion/contraction of space isn't limited by the speed of light (if it were then you could not get places inside black holes' event horizons from which light could not escape)

and what would appear to be violations of causality.

It is not clear whether FTL would actually cause problems with causality (e.g. via Scharnhorst effect)

An issue about FTL (that I think is often overlooked) is that to go from sub-light to faster-than-light speeds one has to cross light speed at some point. That speed is only allowed for massless particles (e.g. photons). So you'd have to convert your spaceship to radiation at some point. Not a good idea.

arom

Bell inequalities are based on the flawed assumption that data from different detector settings can be combined without bothering about the (different) hidden variables of the detectors in their different settings. (There are counter examples for such an assumption and there is a mathematical theorem on it). This has been termed "contextuality loophole", and it can not be closed. Violation of Bell inequalities says at best that quantum mechanics applies, it says nothing about the structure of a hidden variables theory, no statement about absence of locality.

A binary photon structure composed of iteracting charges (providing propulsion) satisfies the required classical dynamics. It also provides a mechanism for the formation of matter: pair formation by threshold gammarays.

No doubts that quantum mechanics rules our world as observed for all experiments, like superfluidity or superconductivity, chemistry, nuclear energy, optics, etc.. Trying to describe quantum reality, with hidden extremely complex and strange classical variables, is trying to extend our macroscopic apparent classical usual misleading experience to domains where clearly it is no more valid !!A photon coming from a quasar, has a wave function of radius of ten billions light years, which on a measurement in a telescop disappears all over this radius instantaneaously, because this single detected photon cannot be detected in another telescop in another galaxy !!You can try any hidden classical variables description, but they will be more strange than quantum description, with an imaginary pilot wave like in Bohm pseudo classical quantum mechanic.The quantum mathematic equations without any other ad hoc hypothesis gives the Everett splitting of univers into parallel worlds .

So you'd have to convert your spaceship to radiation at some point. Not a good idea

Nah, since there's no preferred frame of reference, your spaceship is already moving at the speed of light relative to some frames, and is already converted to photons, ...or your ship is standing still and the rest of the Universe will be converted to photons when it hits the light speed barrier relative to you (I guess that's bad too?)

Anyway, that's kinda sorta just a joke, kinda.

As for the article above, I'm in the unusual position of disagreeing with something that came out of MIT (which probably means that I'm wrong).

Accounts: The only reference I can find is a Google Book page from "The Great Design: Particles, Fields, and Creation" by Robert Kemp Adair, 1989. He gives the "average transition time" as 3 x 10^-9 seconds, which at the speed of light is about 1 meter. Since the transition distance is much less than that, the transition must occur at less than light speed.

That said, I have no idea how accurate his numbers are, so can't vouch for the results.

This CANNOT proove it is not classical as the loophole is still present since the big bang

Yeah, I was wondering about that too. They are proposing a way to make sure the two settings are independent down to a short time after the big bang, but there's still that small slice of time right after the big bang when they weren't isolated from one another. There may even be observational proof that all quasars (and everything else in the visible Universe) are interconnected, since everything we see is biased toward 'normal' and not 'anti' matter. That's fairly compelling evidence that the entire visible Universe has something in common, don't you think?

Rimino

Antimatter does exist. And you don't need two sources that are perfectly uncorrelated - you only need two sources that are not causally linked. And that is a given as they are further apart than their respective horizons.As such positing a partial bias (e.g. there is more matter than antimatter) is not enough to create a loophole. Only positing a full bias would be.

Rimino

Kaiser explains:"A split second after the particles are generated, but just before the detectors are set, scientists would use telescopic observations of distant quasars to determine which properties each detector will measure of a respective particle. In other words, quasar A determines the settings to detect particle A, and quasar B sets the detector for particle B."

The details of exactly how these telescopic observations are accomplished and used were unspecified in the article and that could be an omission that is relevant to the validity of the test. It seems that the telescopic observations themselves being accomplished in the local reference frame of the experiment have the potential to bias the detector settings.

It seems that the telescopic observations themselves being accomplished in the local reference frame of the experiment have the potential to bias the detector settings.

The point is to take some property which isn't dependent on our local reference frame (e.g. whether the fluctuation of the light intensity is on the up or downswing. Or simply how many photons from the source arrive at the detector in a given time interval.).Again: A loophole only exists with full causation - a tiny bit of bias or partial correlationisn't enough to invalidate the findings if the correlation of the experimentally determined values is strong enough.

If we are in a quantum universe anything including faster than light travel is not only possible but probable.

Not really, since that would constitute faster than light information transmission. And that's not in the cards - even in QM.

Well, Bob's particle seems to "know" something about Alice's particle at some point in time. Probably from creation through measurement. How can you NOT call that instantaneous (which we don't even know; maybe it's just MUCH faster than light.) Information transfer?

In reality, Einstein doesn't rule out such transfer at the quantum scale, just at the classical scale.

Or perhaps the human level.

Related? Is there any measurement that determines whether any particle is entangled or not? I don't remember any thing like that. Save for post comparison of the Bell test.

When we say "A affects B", what do we mean by "affect". Yes, it means the outcome of a Bell test will be positive, but only on a statistical basis.

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